Intended for mixed-format material, the 006 has not been heavily used in the past in cataloging books at Houghton. (Manuscript catalogers, because of the nature of the material they work with, have historically relied more heavily on it; see the Manuscript Section's collection-level cataloging manual for details.) However, as more mixed material is being brought into the book collections, catalogers are now encouraged to include it. It particularly should be used whenever accompanying material (subfield e) appears in the physical description (300) area. Examples of this would be books with accompanying CD's, DVD's, or atlases. For detailed instructions on how to fill in the data elements see the MARC documentation at http://www.loc.gov/marc/bibliographic/bd006.html
Though encouraged to do so, catalogers can use their judgment as to when they want to use the 006 to draw out particular elements of a book, for instance, when a book contains maps or music.
For instructions on the use of the Holdings field 007, which is used to describe a feature of a particular holding discrete to that holding only, see Section V.2.